This invention relates generally to coated regenerated cellulose films and, more particularly, to coating compositions that significantly improve the roll formation of regenerated cellulose films.
A major problem associated with vinylidene chloride copolymer (saran) or nitrocellulose coated regenerated cellulose films is poor roll formation resulting in defective finished rolls of film. The defects are most noticeable when processing film of nonuniform thickness such as film having bands, stretched areas, and other gauge-related anomalies and those associated with the processing of a film of uniform thickness under less than ideal conditions; for example, as with excessive and/or nonuniform winding tensions and pressures causing variations in the hardness of the finished rolls. Heretofore, the problem of poor roll formation caused by gauge variations or nonuniform winding conditions has not been satisfactorily solved.
It is well known to improve the slip and blocking characteristics of vinylidene chloride copolymer coatings for regenerated cellulose films by incorporating in the coatings small amounts of finely divided, insoluble, inorganic matter such as bentonite clay, muscovite mica, steatite talc. The particle size of such materials usually falls within the range of 0.5 to 10 microns. Typical such coatings are disclosed in U.S. Pat. No. 3,428,483. While the above coatings improve the slip and blocking characteristics of the coated regenerated cellulose film, they have little, if any, effect upon roll formation.
It is also common practice to dust various film surfaces such as regenerated cellulose film with very fine particles of inorganic material such as talcum powder to overcome matting and slip deficiencies. Nevertheless, such dusting has little, if any, effect upon improving roll formation of the film. A similar technique is also disclosed in U.S. Pat. No. 2,057,548 wherein a comminuted material is applied to the surface of uncoated film to render the film surface less tacky. However, this type of surface application is not practical with coated films.
U.S. Pat. No. 2,909,449 discloses vinylidene chloride copolymer coating compositions to which has been added substantially amorphous vinylidene chloride copolymer particles in two different size ranges, the first from 0.05 to 0.5 microns and the second ranging in size from 2 to 65 microns. Films coated with such dispersions are stated to have a slightly roughened surface and excellent slip and substantially no blocking tendencies. While such coated films when wound into rolls might show some improved roll formation, the amorphous polyvinylidene chloride particles in the coating composition tend to soften in the solution, thereby diminishing the advantages of the particles on the surface of the film and simultaneously diminishing any improvement in roll formation that might be obtained from the larger particle sized materials.
It is also known from U.S. Pat. No. 3,592,725 to make the surface of synthetic resin films suitable for writing or printing by providing a visibly continuous layer of finely divided particles of clay, talc or cellulose pulp on the film, the particles being partially imbedded in the resin film by applying them when the film is in a softened or molten condition. While such surface treatments may be satisfactory with thermoplastic synthetic polymer films, they are not applicable to regenerated cellulose films, and, in any event, have no relation to the problems associated with polymer coatings of regenerated cellulose film.